Hitler's U-boat offensive was on the point of starving the British into submission when, in May 1940, Churchill appealed to Roosevelt for ``forty or fifty of your older destroyers.'' Shogan (Riddle of Power) tracks the tortuous negotiations between Washington and London, with FDR juggling strong domestic isolationist sentiment and the mandates of third-term politics before making a deal in which the British allowed the U.S. use of certain sea bases in exchange for 50 WWI warships. In the end the destroyers did not come up to expectations, and the bases were not as useful as had been hoped; nevertheless, the ships helped stave off Hitler's domination of the Atlantic shipping lanes. In this instructive account of how the president closed the deal without seeking congressional approval, Shogan argues that a pernicious precedent was set, enabling chief executives to override constitutional guidelines under the pretense of protecting national security. Urging total disclosure, increased candor and full accountability, Shogan contends that our presidents can cause harm with foreign policy prevarications: ``Presidential lying is not only immoral, it is also impractical.'' Photos not seen by PW. (Apr.)